The present invention relates to improved methods for synthesizing esters and carboxylic acids from organic alcohols. In particular, it relates to performing such synthesis reactions in the presence of palladium-based catalysts that also include a co-catalyst comprising bismuth, tellurium, lead, cerium, titanium, zinc and/or niobium (most preferably bismuth and tellurium).
Heterogeneous Pd catalysts have been used in organic chemistry for the reduction of some organic compounds. Prominent examples include simple supported catalysts such as Pd/C and Pd/Al2O3, as well as more complex variants such as Lindlar's catalyst, in which Pd/CaCO3 is modified by Pb(OAc)2 and quinoline. Similar catalysts for selective oxidation of organic molecules have been developed, but effective examples are rare.
Adam's catalyst (PtO2) is typically used as a hydrogenation catalyst, but it also mediates aerobic oxidation of primary alcohols to carboxylic acids. High catalyst loading is often required in the latter application (e.g., 0.6 equiv Pt3b), and other reagents, such as the Jones reagent (CrO3/H2SO4) or bleach/TEMPO, tend to be more reliable. Homogeneous and heterogeneous Pd catalysts have been studied for aerobic alcohol oxidation, but this work has mostly focused on conversion of alcohols to aldehydes and ketones. In any event, methods for oxidation of primary alcohols to carboxylic acids and esters are limited with respect to substrate scope (e.g. typically mostly effective with benzylic alcohols) and/or catalyst accessibility.
The literature also refers to homogeneous and heterogeneous Pd based catalysts for the oxidation of alcohols to aldehydes and ketones within the domain of sugar chemistry. Further oxidation of aldehydes to the corresponding carboxylic acid has proved to be challenging with respect to both substrates accessible, or the preparation of the catalyst.
This is particularly of interest as transformation of hydrocarbon feedstocks into value added fine chemicals or pharmaceuticals requires the introduction of various functional groups in the form of a more oxidized molecule. Some reaction conditions/materials have safety drawbacks such as the use of toxic, corrosive, flammable, and/or explosive chemicals, and/or also have low atom efficiency because of stoichiometric byproducts.
The use of O2 for oxidation provides a nontoxic and noncorrosive oxidant alternative with high atom efficiency and water as a benign byproduct. Oxidation of alcohols to aldehydes and ketones has been well studied in the literature, while aerobic oxidation of alcohols to other products such as methyl esters has fewer catalytic studies.
Heterogeneous catalysts have many advantages over homogeneous catalysts including ease of separation and longevity. However, heterogeneous catalysts can be lacking in selectivity, especially towards diverse functional groups present in fine chemicals and pharmaceuticals. Few examples exist of heterogeneous catalysts being able to tolerate wide ranges of functional groups with acceptable selectivity. Most of these examples have limitations such as tedious/costly catalyst syntheses or require specialized equipment for the synthesis.
Another concern is catalysts that suffer from a lack of recyclability and/or longevity.
In H. Kimura et al., Palladium Based Multi-Component Catalytic Systems For The Alcohol To Carboxylate Oxidation Reaction, 95 J. Applied Catalysis A: General 143-169 (1993) there was discussion of multi-component palladium bismuth catalysts (with a variety of different possible additional additives) used to convert certain alcohols to acids. The article did not relate to esterification reactions, and suggested a variety of undesirable limitations even in the context of acids.
Hence, there remains a need for improved methods of forming methyl esters and carboxylic acids, particularly when using primary alcohols as a starting material.